A charge can have only electric field or both electric and magnetic fields at the same time, depending upon, it is either at rest or moving with some velocity. The magnetic field can be uniform or nonuniform depending upon the motion of the charge particle. If charge is moving with uniform velocity the magnetic field is constant with respect to time, but if charge is accelerating i.e.,magnitude of the velocity is changing, the magnetic field will be time varying.

If a moving charge particle having its own electric and magnetic fields passes through external electric and magnetic fields, those external and magnetic fields have their influence on the charge i.e., charge particle will deflects through these fields. The deflection on the charge depends upon many factors like, what is the speed of the charge particle? Angle through which charge particle penetrates into the magnetic field? Mass of the charge particle? Strength of the external magnetic field?

Consider a case when a positive charge q is passing through both the fields at the same time with speed v at an appropriate angle. Consider the figure below, magnetic field is perpendicularly outward to the screen where as an electric field is upward. Both the fields are perpendicular(orthogonal) to each other. In this case the charge particle q will experience an upward electrostatic force and a downward magnetic force. Forces are opposite or antiparrallel to each other keeping in mind that fields were perpendicular to each other.

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If electrostatic force would be greater than magnetic force, charge particle would go upward, if magnetic force would be greater than electrostatic force charge would go downward.These electric and magnetic fields can cancel each other for some particular velocity. When charge particle enters in the orthogonal electric and magnetic fields, the electrostatic force upward is balanced by magnetic force acting on the charge particle downward.

Magnetic force = Electrostatic force

qvBsinθ = qE

If charge eneters in the magnetic field perpendicularly θ= 90⁰

v =E/B

The region having both an electric and the magnetic fields acts as a filter known as Wien filter. It named for Wilhelm Wien who developed it in 1898 for the study of anode rays. It can be configured as a charge particle analyzer

Application of Wien's filter:

Mass Spectrometry:

It is a device that measure masses of ions.Some of the sophisticated designs of spectrometers contains velocity selector method. If velocity, magnetic field, charge can be fixed,radius of the ions is directly proportional to the mass.


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The velocity selector allows only particles(ions) with a particular velocity to get through. In the final region there is a uniform magnetic field and so ions will show deflection according to their masses. Mass spectrometry today is used extensively in chemistry and biology laboratories to identify chemical and biological substances according to their mass to charge ratios.